Pixel buffer circuits for implementing improved methods of displaying grey-scale or color images

ABSTRACT

A device such as a display device or a spatial light modulator can store pixel data in a plurality of small circuits coupled to pixel mirrors and simultaneously drive these pixel mirrors a frame at a time. This device is particularly beneficial for implementing improved image quality techniques which can convert binary images to grey-scale images and/or separate red, green and blue images into color images and displaying those images using the natural process of integration which occurs when a person views images at sufficiently high rates.

RELATED APPLICATIONS

This application is a Divisional of application Ser. No. 09/119,976filed Jul. 21, 1998; which is a Divisional of prior application Ser. No.08/605,999, filed on Feb. 9, 1996, now U.S. Pat. No. 5,959,598.

This application is a continuation-in-part of U.S. patent applicationSer. No. 08/505,654, filed Jul. 20, 1995, now U.S. Pat. No. 5,767,828,the contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to an apparatus and method forimproving image quality and in particular to an apparatus and method forconverting binary images to grey-scale or color images and forconverting a series of red, green, and blue analog images to colorimages, and then either displaying those images or driving a spatiallight modulator.

More specifically, this invention relates to binary and analog framebuffer pixel devices and to frame buffer type devices and methods forimplementing improved methods of displaying images or of driving spatiallight modulators.

2. Background of the Related Art

It has been known that when a person views a rapidly cycled throughsequence of binary images, the person may, if the rate and duration ofimages is proper, temporally integrate such that that sequence of binaryimages and the sequence in turn appears to be grey-scale images. Thisintegration phenomenon is of particular interest with the arrival ofhigh speed binary displays. Such devices are used, for example, inprojection display systems, head-up displays and head mounted displays.There exist small fast high resolution displays which are essentiallybinary in nature such as the Digital Mirror Device (DMD), made by TexasInstruments, active matrix electro-luminescence (AMEL) field emissiondisplay (FED) as well as actively addressed ferro-electric liquidcrystal devices. These technologies are capable of producing manythousands of binary images per second, depending on the number of pixelsper frame, etc . . .

FIG. 1A shows a series of binary images 105 which could be viewed by aperson in the manner described above. Each frame F1-Fm will be comprisedof a series of bits that are either 1 (ON) or 0 (OFF). That is, theseries F1-Fm of frames as well as each individual frame is actually aseries of bits which must eventually be displayed in order to make itpossible for the person viewing the binary images to perform theintegration discussed above. FIG. 1A further shows pixels Pj in general,and P1-P4, in particular, as representative pixels. As each frame F1-Fmis displayed for a time t, some of the pixels Pj will be a logical 1 andsome will be a logical 0. In order for a person to view images producedby frames F1-Fm, a display device is required.

A problem with the above approach is that a display device whichdisplays the group of binary images 105 must be capable of responding inthe time t (which relates to the frame rate 1/t). This places alimitation on which displays can be used. Namely, only those displaydevices can be used which have response rates at least as great as 1/tHz or frames per second. However, the integration process requires thatt be small, otherwise the display would appear to flicker and not appearto provide a grey-scale.

Currently, there are a variety of display devices which might be used tooutput the above discussed subframes. Liquid crystal on silicon (LCOS)devices which have been designed as displays (or spatial lightmodulators) have used pixel designs which can be categorized as beingeither “dynamic” or “static”. A static pixel design has a memory elementat each pixel, which can store the pixel data indefinitely without theneed for periodic refresh cycles. This is analogous to SRAM (staticrandom access memory) in computer memory. A dynamic pixel stores datacapacitively and requires a periodic refresh to compensate for leakageof the stored charge, analogous to DRAM (dynamic random access memory).

Both of these types of displays share the property that as the array ofpixels is addressed in sequence, row-at-a-time, the liquid crystalbegins to update to the new data immediately once the row is addressed.It happens that a reasonably high resolution displays, such as 1024 by1024 pixels, the electronic refresh time is comparable or longer thanthe liquid crystal switching time. For example, if data is supplied tothe display through 32 data wires running at 50M bits/sec, such an arrayof pixels takes approximately 690 microseconds to update. The liquidcrystal switches in approximately 100 microseconds. It is valid,therefore, to view the display as being updated in a sweeping motionacross its area.

In some applications, it would be advantageous to have the data on allof the display be simultaneously valid before it can be usefully viewed.Examples of such applications include most coherent applications such asoptical correlators, optical beam steerers etc . . . , and displayapplications where precise synchronization with other parts of thesystem, such as an illuminated source, is required.

Current pixel designs using liquid crystal displays or microdisplaysfall into two major categories, namely, single transistor pixel systemsand static pixel systems. There are a number of variations to thesetypes of designs, but all relate generally to one of these twoapproaches.

FIG. 1B shows a schematic of a single transistor pixel circuit 701 whichis part of a conventional single transistor pixel array system. Suchsystems are used in the so-called active matrix type computer screens aswell as in some silicon backplane microdisplays which use liquid crystaldisplays. The entire array of pixels is formed such that all of thepixels circuits 701 in a row of the display share a gate wire 705 andall of the pixel circuits in a column share a data wire 710 (or viceversa). Each pixel circuit 701 includes a transistor 714 and a pixelmirror or window electrode 718.

Displays using circuit 701 are updated a row-at-a-time. In particular,gate wire 705 is activated, thereby activating all transistors 714 on asingle row of pixels on the display. Upon activation of gate wire 705,charge flows through transistor 714, thereby bringing the pixel mirror718 to the same voltage as data wire 710. Device 718 can be a pixelmirror, electrode window, or pixel electrode and hence these will beused interchangeably throughout this specification. Gate wire 705 isthen de-activated, thereby trapping the charge and hence the voltage onpixel mirror 718. The voltage on pixel mirror 718 then switches theliquid crystal (not shown). There is a capacitance associated with pixelmirror 718 and the details of the design of such a pixel often deal withmaximizing this capacitance to improve charge storage.

Pixel circuit 701 can be used either as an analog pixel, when thevoltages on data wires 710 are driven to intermediate values, or as abinary pixel when these wires are driven to only two values—typically 0V and 5 V. It must be noted, however, that this pixel display approachis not a frame-buffer pixel as called for in the parent application tothis application. That is, the pixel mirrors 718 are updated arow-at-a-time.

The other type of pixel design that has been used is the so-calledstatic pixel displays. Static pixel displays use pixels which contain adata-latch and possibly other circuitry. This approach has been used,for example, by a research group at the University of Edinburgh inScotland. FIG. 1C shows a schematic of a static pixel circuit 721referred to as a SRAM pixel. Pixel circuit 721 includes a data latch 732connected to array gate wire 705 and data wire 710. Pixel circuit 721also has a pixel mirror or electrode window 718. (Note that gate wire705 and data wire 710 are given the same reference numbers in FIG. 1C asthey had in FIG. 1B.) Here, however, data latch 732 reads the logiclevel on data wire 710 under the control of gate wire 705. A data bit isstored in data latch 732 in the conventional manner that static latchesstore data and hence, the data is stored indefinitely without refresh.Output 740 of data latch 732 can be directly connected to pixel mirror718 or connected to an exclusive-or (X-OR) 750 (as shown) or anexclusive-nor (X-NOR) gate (not shown). Exclusive-or 750 (or the X-NOR)drive a pixel clock (not shown) either in-phase or out-of-phase with aglobal clock line 755 from a global clock (not shown).

X-OR 750 functions in accordance with the signal 740 output from datalatch 732, and consequently functions in accordance with the data bitstored in latch 732. For example, all pixels in the static displaydevice that have a “1” stored in latch 732 take the opposite logic valueof global clock signal 755, whereas all pixels in the static displaydevice that have a “0” stored in latch 732 take the same logic value asthe global clock signal 755. This was originally done to facilitate d.c.balancing of nematic liquid crystals used in earlier liquid crystal onsilicon devices. It has been retained by the Edinburgh group in some oftheir fast ferroelectric devices to assist with frame-inversion, whichis another form of d.c. balancing used with FLC based devices. Hence,once these displays load a frame of data, they have the inverse of thatframe available at the pixel mirrors simply by switching the globalclock.

This pixel display approach is also not a frame-buffer pixel as calledfor in the parent application to this application. That is, although theimage data are stored on the pixel array, the pixel latches 732 (andhence the pixel mirrors 718) are updated a row-at-a-time, just as in thesingle transistor case discussed above. Note that this pixel displayapproach is binary since latch 732 uses restoring logic to pull allnodes in the circuit to either a logic “1” or a logic “0” as does X-ORgate 750.

SUMMARY OF THE INVENTION

Therefore, an object of the invention is to provide a display devicewhich can provide improved image quality from binary or analog displaydevices by updating images a frame at a time.

Another object of the invention is to provide a display apparatus thatcan integrate entire frames of information together before displayingthat information.

Another object of the invention is to provide an apparatus for achievinggrey-scale images produced using binary display devices.

Another object of the invention is to provide an apparatus with one ormore data storage locations at each pixel location.

Another object of the invention is that it includes pixel circuitry thatcan be arranged in a small area about the pixel.

Another object of the invention is to provide an apparatus capable ofproviding an analog signal or binary signal at each pixel.

Another object of the invention is to provide an apparatus fordynamically displaying an image or an apparatus for staticallydisplaying an image.

One advantage of the invention is that is makes it possible to observegrey-scale images using a binary display device.

Another advantage of the invention is that it significantly reduces thetime interval during which the displayed data is changing by avoidingthe row by row updating of the pixels.

Another advantage of the invention is that it can be used to producecolored grey-scale images.

Another advantage of the invention is that it can utilize liquid crystaldisplay devices.

Another advantage of the invention is that it can be used with static aswell as dynamic type display systems.

One feature of one embodiment of the invention is that it utilizesinverters to drive pixel electrodes in one embodiment.

Another feature of an embodiment of the invention is that it utilizescapacitors to store information.

Another feature of the invention is that it can drive the pixelelectrodes with an analog or binary voltage.

Another feature of the invention is that it utilizes only n-FETtransistors in one embodiment.

Another feature of the invention is that in one embodiment the ON pixelsin the least significant frame is displayed at approximately half theirfull duration but no change in their output or ON intensity.

Another feature is that the non-attenuated subframes are groupedtogether to reduce the rate at which the display device outputssubframes.

Another feature on an embodiment of the invention is that the ON pixelsin the least significant frame is displayed at approximately half theirfull or ON intensity.

Another feature of the invention is that it can utilize pixel buffers ora frame/image buffer.

These and other objects advantages and features are achieved by theprovision of a device comprising: a substrate having a first surface; aplurality of driving electrodes arranged on the first surface of thesubstrate; and a plurality of means arranged on the substrate andrespectively coupled to the plurality of driving electrodes, forreceiving image data comprised of a series of subframes and driving theplurality of driving electrodes in accordance with a switching signal.

The above and other objects, advantages and features are furtherachieved when each of the above plurality of means comprises: a firstswitch coupled to a gate signal and a data line for receiving a pixeldatum of the image data and outputting the pixel datum in accordancewith the gate signal; a first inverter coupled to the first switch forreceiving the pixel datum; a second switch coupled to a clock signal andthe first inverter; and a second inverter coupled to the switch and to arespective one of the plurality of driving electrodes, wherein the pixeldatum is transmitted from the first inverter to the second inverter inaccordance with the clock signal, and outputs the pixel datum to saidrespective one of the plurality of driving electrodes.

The above and other objects, advantages and features are furtherachieved when each of the above plurality of means comprises: a firstswitch coupled to a gate signal and a data line for receiving a pixeldatum of the data and outputting the pixel datum in accordance with thegate signal; a capacitance means coupled to the first switch forreceiving and store the pixel datum; a second switch coupled to a clocksignal and the capacitance means; and an inverter coupled to the switchand to a respective one of the plurality of driving electrodes, whereinthe pixel data is transmitted from the capacitor means to the inverterin accordance with the clock signal, and which outputs the pixel data tothe respective one of the plurality of driving electrodes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a series of binary images which could be viewed by aperson in the manner described above. FIG. 1B shows a schematic of asingle transistor pixel circuit 701 which is part of a conventionalsingle transistor pixel array system. FIG. 1C shows a schematic of astatic pixels circuit 721 referred to as a SRAM pixel. FIG. 1D showsschematically the same sequence of binary images shown in FIG. 1A asthey are input to a binary display device. FIG. 1E shows a series ofgroups of m frames. FIG. 1F provides a brief demonstration of theintegration process. FIG. 1G shows an example of how a series of binaryimages which will be arranged into “bit plane” binary subframes which inturn can be displayed to appear to a viewer to be an pixel image with a4 bit grey-scale.

FIG. 2A demonstrates how subframes (such as bit plane binary subframes)can be displayed in different order within a group of subframes, somebeing advantageous over others in various situations. FIG. 2B shows howthe most significant bit frames can be distributed or spread through theentire group of frames.

FIGS. 3A, 3B, and 3C show an approach for rearranging the frames suchthat the display system is not required to run at a rate 1/t in order todisplay the least significant bit (LSB) frame. FIG. 3D shows the stepsrequired to achieve the process shown in FIGS. 3A-3C according to oneembodiment of the invention.

FIGS. 4A, 4B and 4C show another approach achieve a grey-scale effectfor the case where m′=2 (corresponding to FIG. 3C) with a frame rate ofapproximately 1/(4t). FIG. 4D shows a method for displaying a grey-scaleimage on a display unit with a plurality of pixels according to anotherembodiment of the invention.

FIG. 5A shows how 8 bit grey-scale images (or 3×8 bit color images) canbe displayed using a binary display device such as the device of FIG.1F. FIG. 5B demonstrates how analog image signals as well as digitaldata (such as the images of FIG. 5A) can lead to binary subframes whichin turn can be displayed via the methods of FIGS. 3A-3D and 4A-4D.

FIG. 6A shows a display which can serve as display 115 and FIG. 6B showsa close-up view of any one of pixels Hj according to another embodimentof the invention.

FIG. 7A shows a first embodiment of a frame-buffer style of pixeldisplay which uses a CMOS version of a double inverter circuit(corresponding to the buffer circuit in FIG. 6B) for signal storage andregeneration. FIG. 7B shows a second embodiment of a frame-buffer styleof pixel display which uses a CMOS version of a double inverter circuitwith additional transistors for signal storage and regeneration.

FIG. 8 shows another embodiment of a frame-buffer style of pixel displaywhich uses a single inverter.

FIG. 9A shows an analog frame-buffer pixel circuit 901 according toanother embodiment of the invention. FIG. 9B shows a schematic of ananalog frame-buffer pixel circuit 951 that uses only n-FETs and requiresone less transistor and two fewer addressing wires per pixel.

FIG. 10 shows a schematic of a two storage location version of theanalog frame buffer pixel shown in FIG. 9A according to anotherembodiment of the invention.

FIG. 11 shows one such more complex pixel circuit according to yetanother embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Several embodiments of frame-buffer type devices will be discussed.First, however, methods and apparatii for displaying grey-scale or colorimages using such frame-buffer type devices will be discussed withreference to FIGS. 1-5. Then a general buffer type display device willbe presented in FIGS. 6A and 6B which takes advantage of the integrationmethods discussed with reference to FIGS. 1-5. Specific embodiments forbinary or analog buffered displays will then be presented in FIGS. 7through 9, some of which are dynamic (active) type displays, and some ofwhich are static type displays.

FIG. 1D shows schematically the same sequence of binary images 105 shownin FIG. 1A as they are input to a binary display device 115 which hashardware pixels Hj which are either on or off (bright or dark)corresponding to the respective value Pj in frames F1-Fm. Note thatalthough a 4 by 4 pixel display and images are depicted, the followingdiscussion applies to any display and frame size.

Suppose P1 is 1 (ON) for every frame F1 through Fm, P2 is 1 (ON) forframes F1 through Fm−1 and is 0 (OFF) for frame Fm, P3 is 1 ON only forframes F1 and F2 and 0 (OFF) for frames F3-Fm, and P4 is 1 (ON) only forframe F1 and 0 (OFF) for frames F2-Fm.

The rate at which the frames are displayed by display device 115 is 1/tHz, where t is the time between any two consecutive frames Fj and Fj+1.Since P1 is ON for all frames, pixel H1 remains ON for a time mt. SinceP2 is ON for frames F1 to (Fm−1), H2 is ON for a time (m−1)t. Since P3is ON only for frames F1 and F2, H3 is ON for a time 2t. Since P4 in ONonly for frame F1, H4 is ON only for a time t. Integration is achievedas follows. If display device 115 has a quick enough response rate, aperson viewing it notices that pixel H4 is slightly brighter than thosepixels which were not ON at all, i.e., all pixels Pj other than P1 toP4. Similarly, pixel H3 appears slightly brighter than pixel H4 since itis ON for 2t rather than t. Similarly, H1 appears brighter than H2because it is ON for a time mt whereas H2 is ON for a shorter time(m−1)t.

In all of the above statements, it is assumed that the time t is shortenough that a person would not actually see or notice that H4 is ON fortime t and then Off for the rest of the time (m−1)t, whereas H1 is ONfor the entire time mt. Instead, the viewer would integrate the imagestogether which means that to the viewer both H1 and H4 appear to be ON,but H1 is much brighter than H4.

FIG. 1E shows a series of groups 105 of m subframes. Here, the totalnumber of subframes being viewed is N, and again the rate at which eachframe is updated is 1/t where t is the time between frames. Each group105 is integrated by the human eye of the observer viewing device 115 soas to appear as a series 155 of grey-scale images 105′ eachcorresponding to the group of images 105 after integration. Here, msubframes are required to form a single grey-scale (or color) image orframe and N subframes form a sequence of grey-scale (or color) images.

FIG. 1F provides a brief demonstration of the integration phenomenon. Inparticular, FIG. 1F shows intensity output from H1-H4 of I(P1), I(P2),I(P3) and I(P4) versus time for four points P1-P4 under a hypotheticalsituation. The number of subframes is m. The following discussionrelates to the first group 105 of subframes. Pixel H1 is ON for theentire m subframes, H2 is ON during the third sub-frame and off for theremaining subframes, H3 is ON for the first and second subframes and OFFfor the remaining subframes, and H4 is ON for the 5th subframe and offfor the remaining subframes. If the rate 1/t is sufficient such thatintegration occurs in the viewers mind, then the intensity I(Pj) wouldappear to be as follows (intensities are relative intensities. I(P1)=(1,1, . . . 1)→m, I(P2)=(0, 0, 1, 0 . . . 1)→1, I(P3)=(1, 1, . . . 0, 0)→2,and I(P4)=(0, 0, 0, 0, 1, . . . 0, 0)→1. Note that the peak intensity isrepresented by the time sequence (1, 1 . . . , 1) (the lowest intensityis (0, . . . , 0)). Also, note that the intensity at point P2 willappear (if properly integrated) to be the same as the intensity at pointP4 and their order of occurrence is not noticeable. Consequently, thesubframes can be interchanged within a group 105 and provide the samegrey-scale image to an observer when properly integrated by theobserver, and indeed the correct distribution of subframes may aid theprocess of integration.

FIG. 1G shows an example of how a series of 4×4 binary images which willbe arranged into “bit plane” binary subframes which in turn can bedisplayed to appear to a viewer to be a 4×4 pixel image with a 4 bitgrey-scale. Note that although FIG. 1G shows 4×4 pixel images, thetransverse dimensions of the images can be any two integers. Also, thesetransverse dimensions just happen to be the same as the number of bitsof grey-scale which also can be any integer. That is, a 4 bit grey-scaleis shown for discussion and demonstration purposes only.

The group 105 of subframes shown in FIG. 1G are binary subframes whereON pixels are represented by 1 and OFF pixels are represented by 0. Atotal of 2⁴−1=15 such binary subframes 105 are contained in group 105for 4 bit grey-scale images. Also, since this is a 4 bit grey-scale,there need only be 4 bit plane subframes (this number can be increased,if desired). The most significant bit (MSB) subframe shows an image withall pixels that are ON or 1 for at least 8 subframes in group 105. Ascan be seen, only pixels (2, 4) (which (2, 4) is ON in all of thesubframes in group 105) and all of the pixels on the y=1 row, i.e., (1,1), (2, 1), (3, 1) and (4, 1) (which is repeated 8 times). The next mostsignificant bit (the 2²=4) or third bit rearranged into 4 sets ofbit-plane subframes. Only pixel (2, 4) is ON in this example for all ofthese bit-plane subframes. The next to the least significant subframehas two pixels ON, namely, (2, 4) which is ON for all subframes asdiscussed above, and (3, 1) which is ON for the 8 identical subframesand for 2 additional subframes within group 105.

The process of arranging subframes from group 105 into the so-calledbit-plane subframes can be done in a wide variety of ways and isreferred to here as “bit slicing”. One approach is as follows. Thebinary data which represents the stream of binary images could be storedin a computer memory in, for example, a format where an 8-bit byterepresents the grey level to be displayed by a particular pixel (in aparticular color) after integration. One way of generating subframesfrom such a representation is to simply form a 1-bit binary bit-planesubframe from each of the bits of the 8-bit byte. This would be done insoftware by performing a logical AND operation between the byterepresenting the grey and a byte containing all the “0”s except for asingle “1” in the correct position in the byte to extract the desiredsubframe. One hardware implementation could be to read directly thedesired bit for the bit-plane subframe from the stored byte byconstructing the memory hardware in such a way as to facilitateselectable bit-read operations instead of byte-read operations.

One difficulty or potential problem with the above approach is that thedisplay device 115 must be capable of responding to the time t (whichrelates to the frame rate 1/t). This places a limitation on whichdisplays can be used. Namely, only those display devices can be usedwhich have response rates at least as great as 1/t Hz or frames persecond.

The situation discussed with reference to FIGS. 1A, 1C-1F can be used toproduce images with grey-scale in Red, Green and Blue as follows.Suppose that m=100, N=10,000 and t=0.1 milliseconds. These numbers wouldmake available, in one second 100 frames or images, each comprised of100 binary sub-frames (corresponding to frames 105 in FIGS. 1A, 1D, and1E) to generate one grey-scale image for one color. If a complete colorimage is desired, then three grey-scale images (one each for red, greenand blue) would be required. In that case, approximately 32 subframeswould be available for each Red, Green and Blue image if we wish todisplay 100 color images. These 32 subframes can be used to produce 33equally spaced grey levels which is equivalent (approximately) to 5 bitsof grey-scale for each of Red, Green and Blue. This will be discussed inmore detail below.

The above phenomenon makes it possible that the subframes can bedisplayed in any order within a group 105. In addition, some orders ofdisplay of subframes may be advantageous over others as will bediscussed below. Referring to FIGS. 1D-1F, least significant bit (LSB)subframes and most significant bit (MSB) subframes are defined asfollows. A least significant bit (LSB) subframe is defined to be thatsubframe in which pixels may be ON for only one time t within group 105of subframes, thus forming the least significant bit of a binaryrepresentation of a grey-scale image, and a most significant bit (MSB)subframe is defined to be that set of 2^(p−i) subframes which some orall pixels are ON within group 105 of subframes where p is defined asthe integer for which the following holds: 2^(p−1)+ . . .+2^(n)=(2^(p)−1)=m, see FIG. 1F. Hence, the LSB subframe -is that singlesubframe in which the intensity may be ON to contribute the intensitycorresponding to the LSB of a grey-scale image, and the MSB subframe isthat set of 2^(p−1) for which the intensity of a pixel may be ON tocontribute the intensity corresponding to the MSB of a grey-scale image.

Namely, since all of the subframes in each group are integratedtogether, one can display each of the 5 bit planes, i.e., bit 0 (theleast significant bit or LSB), bit 1, bit 2, bit 3 and bit 4 (the mostsignificant bit or MSB) as shown in FIG. 2A. In this scheme, the leastsignificant bit (bit 0) frame is displayed for one frame or time periodt, the next bit (bit frame) for two frames or time 2t, and the mostsignificant bit (in this case for 2^(p−1)t, where p=5) for 16 frames or16t.

In practice, when the frame rates are approaching the lower limits fortemporal integrating, it is advantageous to spread the MSB through theframe which corresponds to group 105 in order to remove contouringartifacts as is known in the art. FIG. 2B shows one way this might bedone. Comparing FIGS. 2A with 2B, it is seen that those pixels which areON for 16 subframes, i.e., for a total time 16t—thereby corresponding tothe MSB or bit 4, they can be turned ON for half of that time or 8t,followed by pixels with bit 3 are ON for 8t, and then re-turn ON the MSBpixels again for the remaining time 8t so that they have been displayedfor the necessary 16t time.

It is apparent from FIG. 2A and 2B that generation of a 24 bittime-sequential grey-scale (or color) images in this way requires a veryhigh speed display, and/or a reduction an image rate (24-bit refers to 8bit grey-scale for each of the three colors used, which would require255 subframes for each color. Namely, display system 115 has to run fastenough to display the least significant frame, i.e., the framedisplaying the LSB.

FIG. 3A corresponds to FIG. 2A and FIGS. 3B, and 3C show a method ofrearranging the frames such that display system 115 is not required torun at a rate 1/t in order to display the LSB. Note that FIG. 3A showsall pixels displaying the same intensity Io and it is only the amount oftime a particular pixel is displayed that results in the grey-scaleeffect. The MSB subframes are those identical subframes containingpixels which are ON to display the most significant bit. The LSBsubframe is the subframe containing pixels which are ON to display theleast significant bit.

FIG. 3B shows how the group 105 is combined to effect a 5 bit grey-scale(for each of Red, Green and Blue) without requiring that display device115 be capable of rates of 1/t. As can be seen, the rate requirement,for display device 115 is reduced from 1/t to 1/(2t). In order tocompensate for the additional time t that the LSB frames are ON, theintensity of pixels in that frame is decreased by half from Io to Io/2.The letter m′ is used to indicate the number of bits which are groupedtogether to yield the LSB time. Hence, referring to FIG. 3A, m′=0 andhence no additional bit is grouped together with the LSB and thus nodecrease in the required rate of performance of display device 115 isachieved. When m′=1, however, the first bit subframes and the 0th bitsubframe are grouped together as shown in FIG. 3B and hence the raterequirement of display 115 is reduced by half to approximately 1/(2t).This reduction is accompanied, however, by a new requirement thatdisplay device 115 be capable of outputting three different intensitylevels, namely Io, Io/2 and 0, rather the two intensities Io and 0 forthe m′=0 case. For a binary display device this may be accomplished bymodulating the illumination light at the appropriate time, or modulatingthe optical output from the display device at the appropriate time.

FIG. 3C takes the process one step further Here, the LSB frames, the 1stbit frames (frames displaying bits in the next to least bit position)and the 2nd bit frames are grouped together. In this case, the raterequirement for display device 115 is reduced by approximately 75% from1/t to approximately 1/(4t). In this case, since the next to leastsignificant bit (bit 1) is ON just as long as the bit 2 frames are ON,their intensity is reduced by half to Io/2. Similarly, since the LSB bitframe is ON just as long as the LSB frame, the intensity of the LSBframe is reduced by half, from Io/2 as in FIG. 3B to Io/4. Hence, inthis case the rate that display 115 must be capable of functioning, isreduced by approximately 75% from 1/t to approximately 1/(4t). For theexample shown in FIGS. 2A and 2B, this means that the 10 kHz frame rateis reduced to 2.5 KHz.

The approach discussed with respect to FIGS. 3A-3C can be generalized asfollows. FIG. 3D shows steps required to generalize the process shownwith respect to FIGS. 3A-3C. In particular, FIG. 3D shows step 310 forreceiving a series of N frames of binary images (each initially to bedisplayed at a rate of 1/t), where N is an integer. Alternatively, ifgrey-scale or color images are received instead of binary images, thenstep 310 is replaced by steps 310 a and 310 b. Namely, step 310 ainvolves receiving a series of grey-scale (or color) images and step 310b involves forming binary subframes representing these grey-scale (orcolor) images.

After either step 310 or steps 310 a and 310 b are performed, ten 320 isperformed. Step 310 involves arranging the series of N frames of binaryimages into n groups of m binary subframes, where m is less than orequal to N. Step 330 involves attenuating the least significantunattenuated subframes within each group of m subframes as well aspreviously attenuated subframes, (if any) by factor of approximately 2.Step 340 involves pairing up the unattenuated frames to yieldapproximately half as many unattenuated subframes and approximatelydoubling thereby, the duration of the attenuated subframes. Please note,however, that by approximately ½ it is meant that the attenuation couldbe anywhere from a few percent to 20 percent or more of half. The exactamount of attenuation (or variation in intensity could be determined bysimply implementing the attenuation process for various amounts ofattenuation and asking observers or viewers which amount of attenuationis most effective. Note that m′ is increased by 1 once step 340 has beencompleted. Step 350 allows one to repeat the last two steps of 330 and340 until the desired frame rate is achieved.

The above process can be continued and m′ increased. For the case of 8bits, (i.e., m from FIGS. 1A, 1D, and 1E is 255), m′ from FIGS. 3A-3Ccan range from 0 to 7. The number of subframes for m=255 is: 255 form′=0, 128 for m′=1, 65 for m′=2, 33 for m′=3, 19 for m′=4, 12 for m′=5,9 for m′=6, 8 for m′=7. The parameter m′ is the number of bits whichhave their illumination attenuated.

The above approach does result in an effective loss of opticalthroughput. That is, there is a data-rate/throughput trade-off which isshown in Table 1. Note that referring to the left part of Table 1(m′=1,2), the optical throughput is slightly reduced for a significantreduction in the frame rate required for a given image-rate

Also note that the relative data rate is shown for two differentsituations. The first calculation corresponds to the timing which isdrawn in FIGS. 3A-3C for clarity. In this case, the time taken todisplay a complete grey scale image is increased slightly with m′. Thiscan be seen if one compares FIG. 3A with FIGS. 3B or 3C in which one canclearly see that the overall data rate is decreased. That is, theattenuated subframes extend further to the right in FIGS. 3B and 3C thanFIG. 3A. Consequently, in practice, a second calculation can be made toadjust the data rate by shortening the frame durations from 2t (FIG. 3B)or 4t (FIG. 3C) to slightly less than that amount to achieve the datarate to perceive the same image rate. The approximate amount ofadjustment can be calculated as follows. If B_(m) is the number ofsubframes for a given m′, and if m is the number of subframes when m′=0,then as subframes are paired in order to go from FIG. 3A to 3B to 3C,they should be shortened by a fraction of about (mt)/[(B_(m)2^(m))t]=m/[(B_(m) 2^(m))], where mt is the duration of the subframes105 with m′=0 and (B_(m) 2^(m))t is the duration of the subframes 105when for m′ not equal to 0.

TABLE 1 (grey-scale level = 256) subframes 255 128 65 34 19 12 9 8 m′ 01 2 3 4 5 6 7 Rel. 100% 99.6% 98% 94% 84% 66% 44% 25% throughput Rel.data 1 0.5 0.25 0.12 0.06 0.03 0.015 0.008 rate (FIG. 3) Rel. data 1 0.50.255 0.13 0.07 0.05 0.035 0.03 rate (con- stant image rate)

The above table is calculated using the steps in FIG. 3D which can besummarized as follows. Starting with the unattenuated subframes, removethe least significant one and attenuate it to half its value andincrease its duration by a factor of two (along with other alreadyattenuated frames). Then the remaining unattenuated frames can becombined into half as many unattenuated, frames. For example, to go fromm′=2 to m′=3 the process is as follows. At m′=2, there are 63unattenuated subframes and 2 attenuated ones. Taking the leastsignificant unattenuated frame, attenuate it by a factor of two (alsoattenuate the two attenuated frames by another factor of two). We nowhave 3 attenuated subframes and 62 unattenuated subframes which areconverted to 31 unattenuated frames of double the duration. This yields34 subframes.

The effective attenuation of the illumination can be achieved in severalways. One approach is to modulate the intensity of the illuminationapplied to the entire display device 115 at the appropriate time.Another approach is to modulate the transmission of an element betweenthe display and the viewer. Another approach to pulse modulate theillumination source which illuminates the display device at theappropriate time to illuminate the attenuated subframes for a shorterduration. Another approach is to use a display device that has thatcapability of simultaneously allowing subframe data to be loaded at therates described above but then to be displayed for a shorter timesimilar to the case of pulse modulated illumination described above. Theillumination sources in some such devices are easier to adjust thanothers.

FIGS. 4A, 4B and 4C show these two approaches for the above discussedcase of illumination modulation corresponding to FIG. 3C (m′=2) with aframe rate of 1/(4t). In particular, FIG. 4B shows intensity modulationas discussed above. FIG. 4C, however, shows an intensity output toachieve the same or nearly the same result. Again, the intensityprofiles are for the source illuminating display device 115. Here, theintensity of all of the bits remains the same and it is their durationwhich is varied. For example, the duration that the pixel source is ONfor the LSB is time t0, which is less than the time 4t shown in FIGS. 4Aand 4B The next to last bit or bit 1 is ON for a time t1 greater that t0but less than 4t (otherwise it would appear as bright as a pixel withbit 2 ON). In particular, the lengths t0 and t1 are adjusted in a mannersimilar to the adjustment of intensity in that t1 is approximately halfof the total time 4t, i.e., t1 is about 2t. Similarly, t2 isapproximately half of t1 and hence approximately one fourth of 4t orsimply t.

FIG. 4D shows a method for displaying a grey-scale image on a displayunit with a plurality of pixels according to another embodiment of theinvention. Step 410 involves receiving a series of N frames of binaryimages each to be displayed at a rate of 1/t, where N is an integer.Alternatively, if grey-scale or color images are received instead ofbinary images, then step 410 is replaced by steps 410 a and 410 b.Namely, step 410 a involves receiving a series of grey-scale (or color)images and step 410 b involves forming binary subframes representingthese grey-scale (or color) images. Step 420 then involves arranging theseries of N frames of binary images into n groups of m binary subframes,where m is less than or equal to N. Step 430 involves shortening theduration of output of the least significant subframes within each groupof m subframes as well as any previously shortened subframes by a factorof approximately ½. Please note, however, that “approximately” ½, meansthat the shortening could be about 50% + of − 20% or possibly more—thiscan be determined by simply implementing the shortening process forvarious amounts of shortening and observing which amount of shorteningis most effective. Note that m′ is in fact increased by 1 once step 440has been completed. Step 450 allows one to repeat the last two steps of430 and 440 until the desired frame rate is achieved.

Display device 115 can include any time-sequential (grey-scale) displaywhether liquid- crystal on silicon, digital mirror devices, etc. . . .Even if the light modulation mechanism is intrinsically capable of veryhigh frame rates, the data rates from the display driving electronics aswell as the display itself should be reduced for reasons of cost andcabling convenience.

All of the above discussion can be applied to color displays whichbriefly discussed earlier. Here, the of light source may be, forexample, 3 separate light sources, namely, a red light source, a greenlight source and a blue light source. These color light sources can be,for example, a red light emitting diode, a green diode, and a bluediode, respectively or a white light source which is sequentiallyfiltered to appear red, green or blue, or a filter between the displayand the viewer which is sequentially switched to transmit red, green orblue. Each of these light sources is treated in a manner analogous tothe above light source for grey-scale. In each of these situations, theoutput intensity is not attenuated in intensity or shortened induration. Color “grey-scale” can be achieved, however, by applyingeither the steps of FIG. 3D for attenuation or the steps of FIG. 4D forduration shortening. This can be achieved for each of the light sources.That is, each of the red, green and blue light sources can be integratedby an observer as discussed above. For example, if the red light sourceoutputs frames as in FIG. 3A with m′=0, then the rate of output can bereduced to approximately ½ by attenuating the intensity of the red lightsource at the pixels in the least significant frame to approximately ½(i.e., from Io to approximately Io/2), and then combining theunattenuated frames in pairs of duration 2t and doubling the duration ofthe least significant frame from t to 2t in the same manner as discussedin FIGS. 3A-3D and in particular in steps 330 and 340. This process canbe repeated (see step 350 in FIG. 3D). This procedure can be done foreach of the red light source, green light source and blue light source

Another example involves applying the method of FIGS. 4A-4D to each ofthe red, green, and blue light sources. For example, if it is desiredthat the red light source output frames as in FIG. 4A (which correspondsto m′=2), then instead of outputting the least significant frames withpixel -outputs of the red light source at Io/4, the duration of theillumination or attenuation of those pixels is reduced by 4 from 4t tot. Similarly, instead of outputting the next to least significant frameswith pixel outputs of the red light source at Io/2, the duration ofthose pixels is reduced by approximately 2 from 4t to 2t as shown inFIG. 4C. This process can be repeated as in steps 450 in FIG. 3D. Thisprocedure can be done for each of the red light source, green lightsource and blue light source. Note that it may be advantages tointersperse red, green, and blue subframes to aid the integrationprocess.

In practice, color displays are typically achieved using a RGB sourcewhere R corresponds to a subframe of pixels which are displaying red, Gcorresponds to a subframe of pixels which are displaying green, and Bcorresponds to a subframe of pixels displaying blue. Then the lightsource is used to output the following subframes. Referring to FIG. 3A,suppose the corresponding series of red subframes, green subframes andblue subframes are arranged as follows:

RRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBB. . . , where each capital letter corresponds to a frame in FIG. 3A andthis example m=31 (recall that m is the total number of subframes).

If each of the red, green and blue sources undergoes the process of FIG.3B via implementation of steps 310-340 one time (so that m′=1), then theleast significant frame (to be attenuated) can be represented by smallletters r, g, and b for red, green, and blue, respectively Using theabove nomenclature, the output during illumination, the red, green andblue sources would be:

RR RR RR RR RR RR RR RR RR RR RR RR RR RR RR rr GG GG GG GG GG GG GG GGGG GG GG GG GG GG GG gg BB BB BB BB BB BB BB BB BB BB BB BB BB BB BB bb

where a space is depicted here only to make clear that two of theunattenuated frames are combined, it being understood that the spacesare analogous to the vertical lines separating frames in FIG. 3B.Typically, the RGB source outputs frames in the sequence RGBRGBRGB . . .. Hence, the above could be output as RR GG BB RR GG BB . . . rr gg bb.As previously discussed, however, the order of the frames may be changedto aid the process of integration. Finally, the above series of framescould also have a shortened duration (as discussed in FIGS. 4A-4CD) ofthe least significant frame as can be shown as follows:

RR RR RR RR RR RR RR RR RR RR RR RR RR RR RR R GG GG GG GG GG GG GG GGGG GG GG GG GG GG GG G BB BB BB BB BB BB BB BB BB BB BB BB BB BB BB B

where a single letter R, G, or B, means that the duration of time thatthe pixel is ON is approximately half as long as the other pixels butthe intensity of those pixels is not attenuated. Here again, the orderof the frames can be altered and still appear the same to an observer.

For m′=2, the above can is combined as follows:

RRRR RRRR RRRR RRRR RRRR RRRR RRRR rrrr ssss GGGG GGGG GGGG GGGG GGGGGGGG GGGG gggg hhhh BBBB BBBB BBBB BBBB BBBB BBBB BBBB bbbb cccc

where a space is analogous to the vertical lines in FIG. 3C, and s, hand c are each half the intensity of r, g and b, and one fourth of theintensities of R, G, and B, respectively.

Again, it may be useful to change order within the group of m frames,the above could be output in a variety of ways including RRRR gggg BBBBrrrr GGGG bbbb RRRR GGGG BBBB BBBB . . . RRRR hhhh BBBB ssss GGGG cccc.Again, the above series of frames could also have a shortened durationof time as discussed above with respect to FIGS. 4A-4D as follows:

RRRR RRRR RRRR RRRR RRRR RRRR RRRR RR R GGGG GGGG GGGG GGGG GGGG GGGGGGGG GG G BBBB BBBB BBBB BBBB BBBB BBBB BBBB BB B

where double letters RR, GG, and BB mean that the duration of the framesis approximately half as long as for the frames RRRR, GGGG and BBBB,respectively (but the intensity is the same). Similarly, the singleletters R, G, and B, have durations of time that is half as long as theframes RR, GG, and BB, and one fourth as long as frames RRRR, GGGG, andBBBB. Here again, the order of the frames can be altered and stillappear the same to an observer. Again, it should be understood that allof the attenuations and shortenings are approximate as discussed above.

FIG. A shows how 8 bit grey-scale images (or 3×8 bit color images) canbe displayed using a binary display device such as device 115 of FIG.1F. Although 8 bit subframes are shown, it should be understood that anynumber grey-scale can be used if the application demands greater orlesser precision. One way this can be done is to generate the sequenceof subframes from bit-frames derived from analog signals. To do this theanalog signal (or signals if R, G, and B have been separated), whichrepresents the brightness of the image on a series of scan lines couldbe sampled with an analog-to-digital converter (ADC). The outputs fromthe ADC then become the binary values for the bit-frames correspondingto the value of the respective ADC outputs. As the analog signal isrepeatedly sampled, the pixels in the bit-frames are assigned values ina sequence which matches the raster scanning pattern used in the analogsignal representation.

FIG. 5B demonstrates how analog image signals as well as digital data(such as the images of FIG. 5A) can lead to binary subframes which inturn can be displayed via the methods of FIGS. 3A-3D and 4A-4D. In theexample shown in FIG. 5B, 8 bit grey-scale or 3×8 bits pixel color arediscussed, it being understood that any number of bits could be used.FIG. 5B involves either: 1) receiving images in analog form at step 553and converting these images into digital image data; or 2) receiving thedigital images directly. Once received, these digital images arerearranged into bit plane subframes at step 567. Again, as discussedabove with respect to FIG. A if the digital images are grey-scale imagesor color grey-scale images, then step 567 involves bit slicing as shownin FIG. 5A. Alternatively, if the digital images are binary subframes,then step 567 involves bit slicing as shown in FIG. 1G. Finally, step577 involves reordering (if desired) the resulting bit plane subframesand displaying those bit plane subframes the proper number of times inaccordance with that bit plane's grey scale bit location. That is, asdiscussed above, if an 8 bit grey-scale is desired, then the MSBsubframe is displayed 2⁷=128 times, the next to the MSB subframe isdisplayed 2⁶=64 times etc. . . . , to the LSB subframe which isdisplayed one time. Recall that there may be advantages in notdisplaying each bit frame (especially for the MSB) all together or insuccession. That is, sometimes, in order to avoid flicker, the MSB andother images can be split up and intermittently displayed.

FIG. 6A shows a display 505 which can serve as display 115 and FIG. 6Bshows a close-up view of any one of pixels Hj. Liquid crystal on silicon(LCOS) displays or spatial light modulators could serve as display. 115.In particular, referring to FIG. 6A, an LCOS display 505 includes a thinlayer of liquid crystal 509 on a silicon substrate 511 which is coveredby a glass window 515. Substrate 511 includes an integrated circuit 520with pixels Hj. Integrated circuit 520 is used to apply an electricfield across the liquid crystal layer 509 in order to reorient theliquid crystal and thereby modulate a light bean that is reflected fromsubstrate 511 as shown in FIG. 5 or in special processes, transmittedthrough substrate 511.

At this point, it should be noted that it is advantageous to update allpixels simultaneously in situations such as in drive schemes whichutilize an electrical modulation of the cover glass transparentelectrode voltage which can facilitate dc balancing. Changes in theelectrical data presented to the pixel electrodes can be synchronizedwith changes to the color glass voltage, thereby maximizing theefficiency of the drive scheme. It is advantageous if integrated circuit520 uses an area which is comparable with, or less than that used byexisting static pixel designs. Standard 1.2 micrometer CMOS design canbe used as it has for existing static pixel designs to yield anapproximately 20 micrometer by 20 micrometer pixel area.

FIG. 6B shows a close-up view of a group of three pixels Hj such as thethree pixels 521 as well as some of the associated electronics accordingto one embodiment of the invention. Note that FIG. 6B is only aschematic representation of several pixels together with theirassociated electronics. In particular, a series of pixel buffers 525 arerespectively coupled to liquid crystal driving electrodes 529 of pixelsH_(j) to integrated electronics 520. The entire group of pixel buffers525 comprise an image buffer 535. A data input 538 receives image datato be eventually displayed.

Display 505 operates as follows. New image data would be received viainput 538 by integrated circuit 520 and stored in frame buffer 535 butnot yet applied to liquid crystal layer 509. This allows the previousimage to be viewed without it being gradually displaced by the new data.Once frame buffer 535 has been completely filled with the new data, thatnew data is simultaneously transferred from pixel storage elements 526to liquid crystal driving electrodes 529.

Note that the above scenario makes it possible to significantly reducethe time interval during which the displayed data is changing. Forexample, consider using a standard LCOS device which has 1024 by 1024pixels, which addresses and begins to update the pixels a row-at-a-time.For such a standard system which includes 32 data wires running at 50Mbits/second, the displayed data is updated in about 655 microseconds.However, display system 505 which replaces the old image data with thenew image data, is limited to the switching time of the pixels and inparticular, of the liquid crystal device, which is about 100microseconds. Note that pixels H_(j) are not necessarily static andindeed at this point a dynamic type pixel approach might be preferable.

The discussion that follows deals with examples of the system shown inFIGS. 6A and 6B (but the circuits are not limited to such a display),and elements shown in those figures will have the reference numeral fromFIGS. 6A and 6B in parenthesis. The discussion applies equally todisplay devices and/or spatial light modulators. That is, all pixelmirrors or pixel electrodes should be considered as elements for drivingdisplay devices such as liquid crystal displays, electro-luminescentdisplays, deformable mirror displays, or as driving elements of spatiallight modulators, or for any other pixel type display.

FIG. 7A shows a first embodiment of a frame-buffer style of pixeldisplay which uses a CMOS version of a double inverter circuit 761(corresponding to buffer circuit 525 in FIG. 6B) for signal storage andregeneration. This version is binary, because it uses inverters that canonly reasonably be expected to drive to 0 V or Vdd (often 5 V). It isalso a dynamic pixel system, because it requires a periodic refresh tomaintain data which is capacitively stored. Note that FIG. 7A includesdashed lines which represent an alternative version of double invertercircuit 761, which will be discussed with reference to FIG. 7B. Thedashed lines are not considered part of circuit 761 in FIG. 7A, but areincluded for reference purposes.

Referring first to FIG. 7A, double inverter circuit 761 operates asfollows. A global clock (not shown) provides a global clock signal online 765 to a transistor 766. When the global clock signal on line 765is inactive, it isolates input 767 of inverter 769 from output 771 ofinverter 776. A frame of new data on data wires 778 (note that data wire778 corresponds to line 538 in FIG. 6B and also note that there is onlyone wire 778 per pixel circuit 761 which is why only one is shown inFIG. 7A and hence only a pixel datum would be present on each such wire778) is loaded into inverters 776 via transistor 781 and input 782 ofinverter 776 of the pixel displaying a row-at-a-time scheme similar tothat discussed in FIGS. 1B and 1C. A single gate wire 779 is activatedwhich sets a row of inverters 776 to the new data value. When gate wires779 are deactivated, the data is stored on the input capacitance ofinput 782 of inverter 776.

Rows of pixels are sequentially addressed in the above manner until allthe pixels of the display have new data on their inverters 776. Theglobal clock is then activated, causing transistor 766 to allow thetransfer of data from output 771 of inverter 776 to input 767 ofinverter 769. This, in turn, transfers the data to output 783 ofinverter 769 which is connected to pixel electrode 718 (whichcorresponds to electrodes 529 in FIG. 6B). Then, the global clock signalon line 765 is deactivated and the pixel datum is safely stored on input767 of inverter 769. A next frame of data is loaded onto inverters 776via data wires 778 and transistors 781.

Pixel mirror/electrode mirror 718 supplies liquid crystal (not shown) ofthe display with charge throughout the switching process of the liquidcrystal at each pixel. This is advantageous because it leads to fasterswitching and more complete switching. This is especially in highspontaneous polarization materials.

It should be noted that circuit 761 uses single transistors 766 and 781to drive inverters 769 and 776, respectively, and hence there may be apossible threshold drop. Consequently, an alternative embodiment will bepresented which uses two more addressing wires and two more transistorsto allow the full voltage swing through the pass gates to the inverterinputs. This alternative embodiment is shown in FIG. 7B.

FIG. 7B shows a second embodiment of a frame-buffer style of pixeldisplay which uses a CMOS version of a double inverter circuit 791 withadditional transistors for signal storage and regeneration. This versionis also binary, because it uses inverters that can only reasonably beexpected to drive to 0 V or Vdd (often 5 V). It too is a dynamic pixelsystem, because it requires a periodic refresh to maintain data which iscapacitively stored.

Referring to FIG. 7B, double inverter circuit 791 operates in a mannersimilar to FIG. 7A. Namely, a global clock (not shown) provides a globalclock signal on line 765 to transistor 766. A second inverted transistor766′, however, receives a logically reversed global clock signal on line765′ (i.e., the logical inverse of the clock signal on line 765). Whenthe global clock signal on lines 765 and 765′ are inactive, they isolateinput 767 of inverter 769 from output 771 of inverter 776. A frame ofnew data on data wire 778 is loaded into inverters 776 via transistors781 and 781′ in accordance with gate wires 779 and 779′, respectively.Input 782 of inverter 776 of the pixel circuit display a row-at-a-timescheme. Gate wires 779 and 779′ are activated which sets a row ofinverters 776 to the new data value. When gate wires 779 and 779′ aredeactivated, the data is stored on the input capacitance of input 782 ofinverter 776.

Pixels are sequentially addressed by rows in the above manner until allthe pixels of the display have new data on their inverters 776. Theglobal clock is then activated, causing transistors 766 and 766′ toallow the transfer of data from output 771 of inverter 776 to input 767of inverter 769. This in turn, transfers the data to output 783 ofinverter 769 which is connected to pixel electrode 718. Then, the globalclock signal on line 765 and the inverse clock signal on line 765′ isdeactivated and the pixel datum is safely stored on input 767 ofinverter 769. A next frame of data is loaded onto inverters 776 via datawires 778 and transistors 781 and 781′.

The above embodiment shown in FIG. 7B has the advantage of avoidingpossible threshold drop, but requires more area per pixel than that ofFIG. 7A. The next embodiment shown in FIG. 8 is even more compact thanthe embodiment of FIG. 7A.

FIG. 8 shows a single inverter pixel circuit 801. Pixel mirror/electrode718, inverter 769, gate wire 779, and other elements are given the samereference numbers as those provided in FIGS. 7A and 7B where possible.Note that inverter 776 in those figures has been replaced by a capacitor805 which stores data while the array is being addressed. This is thesame approach as that described above with respect to FIGS. 7A and 7B.However, circuit 801 does not have a buffer to drive input 767 ofinverter 769. Consequently, capacitor 805 should be as large aspossible. The only disadvantages in making capacitor 805 as large aspossible is the area on the chip it uses. Capacitor 805 does not slowdown the operation of circuit 801, because, typically the capacitance ofdata wire 778 is so large relatively speaking as to render thecapacitance of capacitor 805 (the pixel capacitance) insignificant fromthe point of view of drive load. The capacitance of capacitor 805 dependon a variety of parameters of circuit 801 such as the desired frequencyof frame-wire (or refresh) operations, the rate of charge leakage frompixel capacitor 805 (e.g., possible optically induced leakage), thethreshold voltages of the transistors in circuit 801, and the amount ofarea for each pixel that can be devoted to capacitor 805.

Referring to FIG. 8, circuit 801 operates in a manner analogous to thedouble inverter circuits 761 and 791 as will be explained. As above,global clock (not shown) provides a global clock signal on line 765 to atransistor 766. When the global clock signal on line 765 is inactive, itisolates input 767 of inverter 769 from output 783 of inverter 769. Aframe of new data on data wires 778 is stored on capacitors 805 viatransistors 781 of the pixel in a row-at-a-time scheme similar to thatdiscussed above. Single gate wire 779 is activated which charges a rowof capacitors 805 to the new data value.

Rows of pixels are sequentially addressed in the above manner until allthe pixels of the display have new data stored on their capacitors 805.The global clock is then activated, causing transistors 766 to allow thetransfer of voltage and hence an entire frame of data is transferredfrom capacitor 805 to input 767 of inverter 769. This in turn, transfersthe data to output 783 of inverter 769 which is connected to pixelelectrode 718 a frame at a time. Then, the global clock signal on line765 is deactivated and the pixel data is safely stored on input 767 ofinverters 769 while the next frame of data charges capacitors 805 viadata wires 778 and transistors 781. The data which appears at pixelmirror 718 is of the opposite polarity from the data on data wires 778.

The above discussed circuits were pixel circuit designs which drive thepixel electrodes 718 to binary values. The discussion that follows dealswith circuits that drive pixel electrodes 718 to analog voltages.

FIG. 9A shows an analog frame-buffer pixel circuit 901 according toanother embodiment of the invention. Note that the process ofintegrating subframes is not required for an analog pixel circuit sinceby definition an analog circuit can output grey-scale type images.However, as previously discussed, if an observer sees three separategrey scale images of red, green and blue in series (rather thansimultaneously), he or she will integrate those images together(provided they appear at high enough rates such that the integrationoccurs. This occurs typically at frame rates beginning at approximately180 Hz (3 times 60 Hz) in a pattern of RGBRGB . . . which representschanging a liquid crystal color filter from red (R) to green (G) to blue(blue) or rotating a color wheel or sequential activation of Red, Green,and Blue light sources such as light emitting diodes. In any case, thepixel circuits represented in FIGS. 9A and 9B provide the capability ofswitching frames of analog data an entire frame at a time by capturingan entire frame at a time before displaying that frame. This makes itpossible to precisely synchronize switching from an R frame to a G frameto a B frame rather than trying to synchronize the row-by-row updatingof the prior art displays or spatial light modulators.

Furthermore, these pixel circuits will facilitate the rapid display ofmultiple Red, Green, and Blue within the duration of a single image,which can provide a variety of additional benefits. For instance, in theexample above, one Red, one Green, and one Blue subframe are used toform a single color image which, in this example, lasts for one sixtiethof a second. It is advantageous to intersperse more subframes into thetime allotted for the single color image. For example, six analogsubframes could be used (instead of three) within the {fraction (1/60)}second time period and they could be presented in the order RGBRGB, ornine analog subframes RGBRGBRGB, or twelve analog subframesRGBRGBRGBRGB, ETC . . . . This process can be extended by repeatedlydisplaying groups of RGB's within the duration of time that a singlecolor image would be displayed to achieve visually smooth motion (i.e.,{fraction (1/60)} second). In this approach, all of the Red subframescould be identical, all of the Green subframes could be identical, andall of the Blue subframes could be identical. The above identificationapplies to any order of displaying Red, Green, and Blue subframes andthey need not be displayed as Red followed by Green followed by Blue.

In this approach, the rates that these subframes are displayed is abovethat of ordinary display rates. The advantages of interspersing moresubframes through the time allotted for a single color image are areduction in image flicker and a reduction in color breakup effects inmoving images. The term “color breakup” refers to a phenomenon in whichthe human visual system perceives color fringes around the edges ofmoving objects. It has also been observed that interspersing the Red,Green and Blue is much more effective in reducing image flicker andcolor breakup as opposed to displaying groups of Red subframes followedby groups of Green subframe and Groups of Blue subframes. Again, thepixel circuits discussed above and below provide hardware capable ofachieving such high display rates.

Pixel mirror 718 is driven to the data voltage level through pull-up andpull-down transistors which are clocked as will now be explained.Circuit 901 will be described with the premise that a previous image isalready capacitively stored on pixel mirrors 718. Again, rows of thedisplay are sequentially addressed by activating gate lines 779 and 779′(i.e., line 779 goes high and line 779′ goes low). Data wires 778 thencharge the capacitive input 905 which is the gate of voltage limitingMOSFET 909 to the analog voltage on those data wires 778. This is donefor each row of the display.

Pixel mirrors 718 are simultaneously reset (set to zero volts) by a HIGHon global pull-down line 915 by pull-down transistor 917. This globalpull-down line 915 can be maintained on HIGH for enough time to switchcertain liquid crystal materials if, for example, they have a highspontaneous polarization. Examples of such a liquid crystal material isBDH 764E which requires approximately 30 microseconds to fully switch.As it switches, the reorientation of the molecular electric dipolespartially neutralizes the charge on the pixel electrode. It isadvantageous if the pixel electrode charge can be replenished throughthe time the liquid crystal is switching, so that the chargeneutralization does not cause a perturbation of the voltage on theelectrode, and a corresponding perturbation of the desired “off” state.Another example of a liquid crystal with a permanent dipole is thechiral smectic distorted helix ferroelectric materials made by HoffmanLaRoche. Its characteristic switching time is approximately 200microseconds. All of the pixel mirrors 718 are then simultaneously setto their new analog voltages by the activation of pull-up transistors927, i.e., by setting global pull-up line 925 LOW.

The above happens as follows. Current flows from Vdd line 931 throughpull-up transistor 927 which is switched fully “on” and through voltagelimiting transistor 909 to pixel mirror 718. It must be noted here thatMOSFETS undergo a phenomenon called “pinch-off” which limits the voltagesignal which can be passed by an “on” transistor. Hence, the voltagethat can be passed is limited to the voltage on gate 905 (V_(gate))minus the threshold voltage (V_(th)) of transistors 909. Pixel mirror718 therefore charges up to V_(gate)−V_(th), thereby allowing thepreviously set gate voltage to control the voltage pixel mirror 718charges up to.

In a standard CMOS process, the n-transistor threshold is a positivequantity and so pixel mirror 718 cannot be charged up completely to thesupply voltage Vdd.

FIG. 9B shows a schematic of an analog frame-buffer pixel circuit 951that uses only n-FETs and requires one less transistor and two feweraddressing wires per pixel. Hence, this design is more compact than thatshown in FIG. 9A. Using only n-channel transistors removes the need foran n-well at each pixel as well as a power supply rail to clamp the wellvoltage. However, this design does have another threshold voltage drop.Again, identical reference numbers are used for those elements ofcircuit 901 (FIG. 9A) which are common to circuit 951.

Referring to FIG. 9B, pass gate 781 and 781′ is replaced with a singlegate 781. Also, p-type pull-up transistor 927 has been replaced by ann-type transistor 967. Here, data voltage is transmitted directly tovoltage limiting MOSFET 909 through only n-type transistor 781. Hence,the maximum voltage that can be transmitted to gate 905 isV_(gate)−V_(th) where V_(gate) and V_(th) are the same as defined above.This in turn means that the maximum voltage which can be transmittedthrough voltage limiting transistor 909 is Vdd−2V_(th). It is possibleto arrange for the transistors in circuit 951 to have a low (pehaps afew tenths of a volt) threshold voltage V_(th) by including an extramask so that selected transistors are processed to have a different(here lower) threshold.

Other more complex implementations of pixel circuits can be made in viewof the above discussion. One such complex implementation involvesextending any of the previously described circuits to have more than onestorage location at each pixel. This can be done by having more than onedata wire going to each pixel, and simultaneously clocking data ontomore than one storage location under the control of a single gate wire.Alternatively, each pixel can have a single data wire and more than onegate wire to control which storage location the data present on the datawire is clocked onto. The formatting of the input data would determinewhich approach is preferable.

A multiple storage location pixel also requires a mechanism fordetermining which storage location is used to control the pixelelectrode at a given time. This might require extra transistors andcontrol wires at each pixel, thereby increasing its complexity andphysical size. This type of complex pixel may be advantageous forswitching rapidly between images such as Red, Green and Blue images asdescribed above or for performing data reformatting such as parallel toserial conversion if data arrives on several wires to the pixel, but isread out in serial.

A schematic of a two storage location version of the analog frame bufferpixel shown in FIG. 9A is shown in FIG. 10. This schematic is a multiplestorage location frame buffer pixel with two storage locations and isbased on the pixel circuit in FIG. 9A.

The circuit in FIG. 10 operates the same as described for FIG. 9A exceptthat data is simultaneously presented on both data wires 778 and 778′,and simultaneously clocked onto the gates of transistors 909 and 909′.Either pull-up transistor 927 or pull-up transistor 927′ is activatedduring the driving sequence, thereby selecting which storage locationcontrols the pixel voltage.

FIG. 11 shows one such more complex pixel circuit 1001 according to yetanother embodiment of the invention. Here, several bits of digital datacan be stored at each pixel and converted locally to an analog signalfor driving mirror/electrode 718. Circuit 1001 includes a data latch1005 which is a n-bit data latch coupled to one or more data wires 778under the control of gate wire 779. Once the data is loaded onto datalatch 1005, switch 1009 is activated with global clock signal 765 andthe data bits are simultaneously transferred to the digital-to-analogconverter (DAC) 1014 which drives pixel mirror electrode 718 to thedesired voltage. This approach could easily be extended to incorporateautomatic dc balancing circuitry such as the XOR circuit discussed withrespect to the SRAM pixel.

The approach of FIG. 11 requires a larger number of transistors forcircuit 1001 than the circuits discussed above. For that reason, itwould be unlikely that circuit 1001 would be preferable for mostdisplays, because often it is desirable to put as many pixels aspossible in a given area of silicon. However, circuit 1001 and othercomplex circuits may be advantageous for specialized applications suchas optical wavefront correction where it is typically not as importantto have a large number of pixels, but instead it is more important toaccurately control their optical state.

What is claimed is:
 1. A method for displaying a color image on adisplay unit with a plurality of pixels, comprising the steps of:receiving a Red frame, a Green frame, and a Blue frame comprising thecolor image to be displayed within a first time period corresponding toa first frame display rate; and repeatedly and interspersedly displayingsaid Red frame, said Green frame and said Blue frame within the firsttime period at a higher rate than said first frame display rate.
 2. Themethod for displaying a color image as claimed in claim 1, wherein saidrepeatedly and interspersedly displaying step comprises interspersedlydisplaying said Red frame, said Green frame and said Blue frame twotimes each, within said first time period.
 3. The method for displayinga color image as claimed in claim 1, wherein said repeatedly andinterspersedly displaying step comprises interspersedly displaying saidRed frame, said Green frame and said Blue frame three times each, withinsaid first time period.
 4. The method for displaying a color image asclaimed in claim 1, wherein said repeatedly and interspersedlydisplaying step comprises interspersedly displaying said Red frame, saidGreen frame and said Blue frame N times each, within said first timeperiod, where N is an integer which is greater than or equal to two. 5.The method for displaying a color image as claimed in claim 1, whereinsaid receiving step comprises receiving the Red frame, Green frame, andBlue frame comprising the color image to be displayed within a firsttime period which is less than or equal to about {fraction (1/30)} of asecond.
 6. The method for displaying a color image as claimed in claim1, further comprising repeating said receiving step followed by saidrepeatedly displaying step for a subsequent color image.
 7. The methodfor displaying a color image as claimed in claim 1, further comprisingrepeating said receiving step followed by said repeatedly displayingstep for each color image within a series of subsequent color images. 8.The method for displaying a color image as claimed in claim 1, whereinsaid repeatedly and interspersedly displaying step comprises repeatedlyand interspersedly displaying said Red frame, said Green frame, and saidBlue frame in any order.
 9. A method of displaying a color image on adisplay unit with a plurality of pixels, comprising the steps of:receiving a first color frame, a second color frame, and a third colorframe comprising the color image to be displayed within a first timeperiod corresponding to a first frame display rate; and repeatedly andinterspersedly displaying said first color frame, said second colorframe and said third color frame within the first time period at ahigher rate than said first frame display rate.
 10. The method of claim9, wherein said first, second and third color frames are each displayedon at least a row of pixels.
 11. The method of claim 9, wherein saidfirst, second and third color frames are each displayed on at least tworows of pixels.
 12. The method of claim 9, wherein said repeatedly andinterspersedly displaying step comprises interspersedly displaying saidfirst color frame, said second color frame and said third color frame Ntimes each within said first time period, where N is an integer greaterthan or equal to two.